"iFormula" is a free application from the Apple Apps Store for iPhone, iPad, and iPod touch.
https://itunes.apple.com/us/app/iformula-your-own-calculator/id665486714
I wanted a tool where I could record several formulas I had learned of from a KD shoot, and keep them handy rather than buried in notes. The app works well and instantaneously, giving the shooter the ability to set up input fields for variables, set constants and formulas, as well as provide a description of the formula.
I have created the following formulas: Windage & Factor>Hold in MOA, Temperature-change>Hold in MOA, Slope>Hold at given range, Observed target in MILS>Range Estimation, Observed target in MOA>Range Estimation, MILS>MOA, News-Barometric-pressure-to-equivalent Station-Pressure (for ballistics programs), MILS>Inches-at-a-given-range, MOA>inches-at-a-given-range
msc
Nice find! Useful for anything you need to calculate on a regular basis.